Phytophthora
Phytophthora cinnamomi, also known as Jarrah dieback and Phythophthora dieback is a pathogen that attacks the root system of plants.
Phytophthora dieback is common in Western Australian home gardens, parks and nurseries and is readily transported in infected soil which may stick on shoes or vehicle tyres.
Once plant roots are infected, the pathogen produces mobile zoospores at an optimum temperature of around 15°C. These swim in water to the root tips of other plants which they may infect. In warm, wet conditions such as heavy rainfalls in summer, the fungus can complete its life cycle within 24 hours. Little infection takes place below 15°C.
There are more than 50 species of Phytophthora and they cause a variety of symptoms, depending on the species attacked and the soil conditions where it grows. Plants infected with the disease may suddenly collapse and die within a few days after having been apparently healthy. About 40% of native WA plants are suseptible. Plant collapse is common in susceptible dry climate plants like many Banksia spp., Dryandra spp., Adenanthos spp., Isopogon spp., Hakea spp., Grevillea spp. and Eucalyptus spp.
The collapse is thought to be the result of the breakdown of the plant’s root system. Plants may have survived for a long time as the remaining functional root system supplied sufficient nutrients and water under mild conditions. However, in period of high evaporative demand, the damaged roots supply insufficient water to maintain effective transpiration and cooling.
Phytophthora becomes more of a problem in waterlogged soil, and only resistant plants survive.
In the well drained soils of most Perth metropolitan gardens, Phytophthora may not kill the plants but may cause stunting, slow growth and affect the growing tips. The small feeder roots are attacked and the leaves are often yellow with brown margins and apices. After warm, wet periods of 24 hours or longer, the fungus may cause sufficient damage to the roots of the plants to kill them.
Cultural control can be achieved by improving the soil drainage through installation of underground drains, raised beds and sunken garden paths. In areas of known infection, only tolerant species should be grown. Sowing infected areas to grass for at least three years should reduce the chance of fungal re-infection.
The home gardener can achieve chemical control with phosphorous acid (also known as phosphite) which can be sprayed onto the foliage of plants or injected into trees.
Pythium, Fusarium and Rhizoctonia
These are damping off diseases which affect seedlings all year round, but mainly in wet conditions. Seedlings die before they emerge or after emergence. In the latter case, rot develops on the stems and constricts water and nutrient movement near the soil surface. The stems are physically weakened and the plants collapse.
To avoid damping off disease, do not over-water. Sow seeds into sterilised soil thinly, to avoid crowding of the seedlings. Pythium may attack plants which have soft tissue due to an over-use of nitrogenous fertiliser. Treat with sulphur and mancozeb.